MarIus

2.1.1 The Due Diligence Standard in International Environmental Law

The due diligence standard in international law requires States to take all appropriate measures to prevent transboundary or common-area harm from activities under their jurisdiction or control. It is a standard of conduct rather than of result: it does not require that no harm occur, but that the State exercise the level of care that a government acting diligently would be expected to deploy in similar circumstances.(1) Malaihollo, “Due Diligence in International Environmental Law,” 121–55; Crawford, Brownlie’s Principles, chap. 20. The standard is graduated to the severity and likelihood of potential harm: where the risk of significant environmental damage is high, the diligence required increases correspondingly, calibrated to the best available scientific standards at the time.(2) Sands and Peel, Principles of International Environmental Law, 4th ed., 603–607; Riccardo Pisillo Mazzeschi, "The Due Diligence Rule and the Nature of the International Responsibility of States," German Yearbook of International Law 35 (1992): 9. The general relationship between due diligence and State responsibility is addressed in the broader international law literature.(3) Birnie, Boyle, and Redgwell, International Law and the Environment, 3rd ed., 137–42; Dupuy, “Due Diligence,” 369; Crawford, Brownlie’s Principles, chap. 20.

The due diligence standard in international environmental law has been progressively established through a series of landmark judicial and advisory opinions. The foundational expression of the no-harm rule, that States may not use or permit use of their territory in a manner causing injury to another State or to areas beyond national jurisdiction, emerged from the Trail Smelter arbitration(4)Trail Smelter Case (United States v. Canada), 3 R.I.A.A. 1905 (1938, 1941). and is now reflected in Principle 21 of the Stockholm Declaration (1972) and Principle 2 of the Rio Declaration (1992). The ICJ subsequently confirmed the general obligation of States to ensure that activities within their jurisdiction and control do not damage the environment of other States or of areas beyond national control.(5)Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, I.C.J. Reports 1996, p. 226, para. 29. The ITLOS Seabed Mining Advisory Opinion (2011) confirmed that the due diligence obligation extends to States sponsoring activities in the Area, requiring the standard of conduct necessary to prevent marine environmental harm and establishing the obligation to conduct EIAs as a component of that duty.(6)Responsibilities and Obligations of States (ITLOS), paras. 110, 115, 141.

The most consequential judicial development is the ICJ's holding in Pulp Mills that the obligation to conduct an EIA before undertaking activities with the potential for significant transboundary environmental harm has "now become a requirement under general international law."(7)Pulp Mills, I.C.J. Reports 2010, para. 204 That holding is not contingent on any specific treaty regime and applies wherever activities carry the potential for significant environmental harm. It has been confirmed and extended in subsequent jurisprudence: Certain Activities (ICJ, 2015) affirmed the obligation's application to cumulative impacts; and the ITLOS Climate Change Advisory Opinion (2024) held that the due diligence obligation under UNCLOS Article 194 requires States to take all necessary measures, including procedural steps such as EIA, to prevent, reduce, and control climate change-related harm to the marine environment.(8)Obligations of States in Respect of Climate Change, Advisory Opinion, I.C.J. Case No. 187 (July 23, 2025).

2.1.2 ‘Jurisdiction or Control’ as a Common Organising Concept Across UNCLOS, BBNJ, and the OST

A critical point of convergence between the three applicable legal frameworks remains that all three employ some variant of a “jurisdiction or control” nexus to attribute regulatory responsibility to States. Through this lens, the frameworks can be interpreted as sharing a common structure of attribution even if their substantive obligations differ.

Under UNCLOS Article 192 and Article 194(1), States are required to take measures to prevent marine pollution in relation to activities “under their jurisdiction or control.” The BBNJ’s Article 6 and Article 28(1) employ the identical phrase. The OST’s Article VI allocates international responsibility to States for national space activities, whether by governmental or non-governmental entities,(9) Outer Space Treaty, art. VI. a responsibility that is operationalised through the Article VIII registration-jurisdiction nexus, which confers on the State of registry "jurisdiction and control" over a space object.(10) Outer Space Treaty, art. VIII; Registration Convention, art. II. The precise scope of this allocation, and in particular the meaning of "national activities" and the identification of the "appropriate State," remains a matter of scholarly debate.(11) Cheng, “Article VI,” 7, at 15; Von der Dunk, “The Origins of Authorisation,” 3–28; Hobe et al., Cologne Commentary, vol. I, art. VI. The Liability Convention and the Registration Convention further specify that the "launching State", the State that launches, procures the launching of, or from whose territory a space object is launched, is the State that bears this jurisdictional responsibility. In all three frameworks, therefore, the critical question is not which treaty applies in the abstract, but which State exercises effective jurisdiction or control over the activity in question; that determination will in most cases yield the same answer across all three instruments simultaneously.

The ITLOS Seabed Mining Advisory Opinion is instructive here. The Tribunal held that the sponsoring State’s obligation of due diligence, including the obligation to require EIAs, applies by virtue of the jurisdictional nexus between the State and the activities of the entities it sponsors. This reasoning maps directly onto the splashdown context: the State that licenses a commercial launch operator, approves its reentry plan, and issues a splashdown authorisation is exercising jurisdiction or control over that activity within the meaning of all three frameworks. Thus, the obligation of due diligence, including the EIA obligation, follows from that jurisdictional nexus, regardless of whether the obligation is grounded in UNCLOS, the BBNJ, or customary international law independently.

2.1.3 The EIA Obligation: Content and Standard

The content of the EIA obligation as a matter of customary international law has been progressively elaborated through judicial decisions and State practice.(12) For additional legal analysis, see Neil Craik, The International Law of Environmental Impact Assessment: Process, Substance and Integration (Cambridge: Cambridge University Press, 2008) While Article 206 of UNCLOS establishes the foundational obligation requiring States to assess potential effects of planned activities that may cause substantial pollution or significant and harmful changes to the marine environment, its normative strength remains modest. The text does not specify a standard of "reasonable grounds," offer threshold criteria, or prescribe procedural frameworks for conducting the assessment. Pulp Mills established the minimum requirements: the EIA must be conducted before the activity is authorised; it must assess the potential significant transboundary or common-area impacts; and it must be disclosed to affected parties. Subsequent decisions have added further content: the obligation extends to cumulative impacts,(13)Certain Activities (Costa Rica v. Nicaragua), I.C.J. Reports 2015, para. 104. the EIA must use the best available scientific evidence and techniques;^{(14)Request for Advisory Opinion (ITLOS, Case No. 31, 2024).} and where scientific uncertainty exists, the precautionary approach requires that the absence of full scientific certainty not be used as a reason to postpone preventive measures.(15) Rio Declaration on Environment and Development, Principle 15, UN Doc. A/CONF.151/26/Rev.1 (vol. I) (1992); BBNJ, Preamble. These customary requirements are, however, general in character: they leave States wide discretion as to the specific content and procedure of any given assessment, and impose no institutionalised mechanism for ensuring transparency or accountability.(16)Pulp Mills, para. 205; Tanaka, "Reflections on the EIA in the BBNJ," 92–93.

The BBNJ's Part IV framework addresses this deficit directly. As Tanaka has observed, Article 27(a) explicitly designates the operationalisation of UNCLOS's EIA provisions in ABNJ as an objective of Part IV, and Article 28(1) constitutes "an important innovation" in placing a clear, treaty-based obligation on parties to ensure that the impacts of planned activities under their jurisdiction or control in ABNJ are assessed before authorisation.(17) Tanaka, "Reflections on the EIA in the BBNJ," 92, 95; BBNJ, art. 27(a); art. 28(1). The BBNJ does not create the EIA obligation ex novo; rather, it gives the pre-existing customary obligation specific and procedurally complete content, elaborating the EIA process across six discrete stages, from preliminary screening through scoping, impact assessment, publication, decision-making, and monitoring, and subjects each stage to oversight by the Scientific and Technical Body.(18) BBNJ, arts. 28–37; This is the basis on which this thesis argues that the BBNJ constitutes the primary applicable framework for splashdown EIAs: it provides, in addition to UNCLOS and customary law, the most specific and institutionally robust expression of the obligation available to States conducting activities in ABNJ.